A thyristor-switched capacitor (TSC) is a reactive power compensation device that is often used in power systems. In a power system that uses a TSC, a branch of capacitor banks can be connected or disconnected to a power grid simply by switching a thyristor associated with the branch, thus providing reactive power injection capabilities.
Two topologies that are often used are the “Delta” type TSC and the “Wye” type TSC. For both topologies, each phase of a transmission system is coupled to a capacitor that provides reactive power compensation. In series with that capacitor, there are two thyristors connected in parallel but in opposite directions, and a surge inductor. The anti-parallel thyristors allow current to flow in either direction, and the surge inductor is used to limit large transient currents through the branch.
When a TSC is turned off, a certain amount of electric charge will be trapped on the capacitor because the AC voltage across the capacitor and the AC current flowing through it are not in phase, i.e. they do not cross zero at the same time. As such, when the TSC is turned off, a constant voltage is left on the capacitor. In a given system, the peak value of this trapped voltage can reach or exceed 2 per unit (pu) values of the nominal system voltage, depending on the type of TSC used and the system's operating conditions. Consequently, a thyristor must be rated with a high blocking voltage in order to prevent damage. This increases the per-component cost and may also require additional protection circuitry, which in turns increases the complexity and overall cost of a TSC system.